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My older (FC11) install had reached EOL, and I tried to reinstall FC14 on its RAID1 root filesystem.

I'm now having suspicions that now, after the install FS is not fully raided. The question is if this is suspicion is true, and if so, how to fix it.

[root@atlas dev]# cat /proc/mdstat
Personalities : [raid1]
md127 : active raid1 sda[0]
  732571648 blocks super external:/md0/0 [2/1] [U_]

md0 : inactive sdb[1](S) sda[0](S)
  4514 blocks super external:imsm

unused devices: <none>
[root@atlas dev]#

md127 seems to be some child container of md0, but lists sda[0] as explicit device, but not sdb. I assume I'm running off sda reading this, and sdb is defunct.

The trouble is that the FS has seen quite some action since, so both discs can't be assumed to be in sync. sdb must probably be rebuilt. I do have a full backup though, so I'm willing to take calculated risks.

Note that the filesystem is the root device. (single user mode?)

Any explanations how to read mdstat output is also welcome. My guess is that I need to somehow add sdb from the md0 container to md127.

Kernel excerpt:

          dracut: Starting plymouth daemon
          dracut: rd_NO_DM: removing DM RAID activation
          pata_marvell 0000:03:00.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
          pata_marvell 0000:03:00.0: setting latency timer to 64
          scsi6 : pata_marvell
          scsi7 : pata_marvell
          ata7: PATA max UDMA/100 cmd 0xcc00 ctl 0xc880 bmdma 0xc400 irq 16
          ata8: PATA max UDMA/133 cmd 0xc800 ctl 0xc480 bmdma 0xc408 irq 16
          dracut: Autoassembling MD Raid
          md: md0 stopped.
          md: bind<sda>
          md: bind<sdb>
          dracut: mdadm: Container /dev/md0 has been assembled with 2 drives
          md: md127 stopped.
          md: bind<sda>
          md: raid1 personality registered for level 1
          md/raid1:md127: active with 1 out of 2 mirrors
          md127: detected capacity change from 0 to 750153367552
           md127: p1 p2
          md: md127 switched to read-write mode.

output of --detail --scan:

 ARRAY /dev/md0 metadata=imsm UUID=e14582dd:1863c14a:fb0d98f0:5490080b
 ARRAY /dev/md127 container=/dev/md0 member=0 UUID=c0cf0b37:bc944eec:ac93d30e:ee2f423e


 # mdadm.conf written out by anaconda
 AUTO +imsm +1.x -all
 ARRAY /dev/md0 UUID=e14582dd:1863c14a:fb0d98f0:5490080b
 ARRAY /dev/md127 UUID=c0cf0b37:bc944eec:ac93d30e:ee2f423e


After waiting for the better part of the day I bit the bullet, verified my backups, booted to single user mode, and there I could simply mdadm --manage /dev/md127 --add /dev/sdb

Rebuilding took about 3 hours (of unpaid overtime). Everything seems to work and look intact.

I also remember I've meddled with fakeraid before deciding to go to software RAID, though afaik on other discs. Maybe the md0 is a left over from that, something that slipped in by an poorly restored /etc and then beat till it worked. The next reinstall it blew up in my face though. The experimenting to get it working probably reserved some info for it.

The scary thing is that both arrays contain the same discs now, AND that md0 is briefly enabled during boot. I get warnings that seem to signal that md127 is a child of md0, which makes deletion a bit scary. But I'll dig a bootdisk and give it a whir the next day I've time for system administration. (after making an incremental to yesterdays full backup of course)

md127 has two partitions ( a big root + swap), both mounted. md0 is not active (and I don't dare to activate it since it shares drives), so I don't know what partitions it has.

Since md127 now works (2/2 UU), it now is a matter of figuring out if md0 can be safely deleted (md127 child of md0?) and if so how, to avoid problems during future installs.

Probably need to kill some of the metadata on disk too, to avoid the next install picking it up.

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Could you provide output of mount, fdisk -l /dev/md0 and fdisk -l /dev/md127 This should let determine what's mounted where, you have some non-trivial setup here ;) –  Paweł Brodacki Apr 28 '11 at 11:41
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1 Answer

up vote 1 down vote accepted

The right thing to do is to first figure out which partition is mounted. Then you'll need to deactivate the other one so you can remove the disk from it. And then grow the mounted disk under mdadm to add the new disk to it, and the md software will sync the currently mounted partition to the other disk. After that cat'ing /proc/mdstat should show only a single md device with both disks listed (and likely one syncing still). (and fix the fstab possibly)

I won't repeat a lot of what is needed here, since the linux raid wiki is quite good and has most of the documentation you need to read (and with good examples).

However, the one thing most people forget is that the linux kernel needs a initramfs that contains the needed disk info. So any time you change the md configuration it is wise to re-create the initramfs using dracut:

dracut -v --mdadmconf new_image.img KERNEL-VERSION-STRING

And then point grub at the new_image.img.

I've made the mistake before of forgetting this after removing a drive from an array and ended up booting off of the single drive, rather than the properly mirrored drive. A very similar situation to what you seem to be in.

share|improve this answer
Linux raid wiki (raid.wiki.kernel.org) is not loading for me. Anyway thanks for the ramfs bit. I'm on a timebudget here, since I'm stuck with a down server. I didn't dare try to remove the md0 in case the md127 would be killed. –  Marco van de Voort Apr 28 '11 at 15:22
Hmm.... You're right that the wiki appears down (I was just looking at it like 2 days ago, so it must be temporary). –  Wes Hardaker Apr 28 '11 at 18:17
It's up now, but still very slow. But I had to do something about it yesterday, so I gambled and it partially worked (see update of question). Will need another session to try to do cleanup so this doesn't happen again. –  Marco van de Voort Apr 29 '11 at 10:17
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